In the course of time, different signs appear on the skin, which are very characteristic of ageing, reflected especially by a change in the structure and function of the skin. This ageing, which is physiological in nature, may be accelerated by environmental factors such as repeated exposure of the skin to sunlight, and especially to ultraviolet A rays, to pollution, in particular either to diesel particulates or to cigarette smoke. The action of the environment on the constituents of the skin (including fibres, cells and enzymes) and on the sebum secreted by the skin leads in particular to the formation of oxygenated free radicals. These radicals cause considerable oxidative damage, especially in cell membranes (e.g., membrane permeability), cell nuclei (e.g., destruction of DNA) and tissues, in particular connective tissue (e.g., degradation of the elastin and collagen fibres). This damage leads especially to a loss of firmness and elasticity of the skin.
It has also been suggested that free radicals might be involved in the process of melanin manufacture leading to pigmentation of the skin.
The mechanism of melanin formation is particularly complex and schematically involves the following main steps:
 Tyrosine→Dopa→Dopaquinone→Dopachrome→Melanin
Tyrosinase (monophenol dihydroxyl phenylalanine: oxygen oxidoreductase EC 1.14.18.1) is the essential enzyme involved in this sequence of reactions. It especially catalyses the conversion of tyrosine into dopa (dihydroxyphenylalanine) by virtue of its hydroxylase activity. However, certain authors think that the step of hydroxylation of tyrosine to dopa might be initiated by OH. radicals (C. Montastier et al., Méthodes d'objectivation des effets des agents dépigmentants chez l'homme, [Methods for evaluating the effects of depigmenting agents in man], J. Med. Esth. and Chir. Derm., Vol. XXII, 86, June 1995, pp. 93-103), incorporated herein by reference.
The free radicals formed due to the effect of environmental factors thus may lead to an increase in the formation of melanin, and thereby cause or accentuate certain undesirable hyperpigmentations such as idiopathic melasmas, occurring during pregnancy (“pregnancy mask” or chloasma) or during oestro-progestative contraception, or alternatively localized hyperpigmentations, such as senile pigmentation marks known as actinic lentigo. It is thus necessary to protect the skin against these free radicals.
This protective function is normally provided by enzymes present in skin tissue. However, in certain circumstances, the role of these enzymes is insufficient to totally block the destructive action of free radicals.
It has thus been proposed to use various natural and synthetic products as cosmetic active agents for combating the formation of free radicals. Among the agents that may be mentioned are:                free-radical-scavenging compounds such as vitamin C, vitamin E and derivatives thereof, and also carotenoids;        compounds which stimulate the enzymes that block free radicals, such as compounds that are rich in phosphorus and trace elements;        compounds that maintain the integrity of the hydrolipid film, having a self-protective function against free-radical-generating radiation. It is in particular gamma-oryzanol, extracted especially from rice bran oil.        